1. Field of the Invention
The present invention relates to an apparatus for heating semiconductor substrates contained in a furnace core tube to perform desired processing on the substrates. The apparatus may be an oxidation furnace, a diffusion furnace, a reaction furnace in a CVD (chemical vapor deposition) apparatus and the like. More specifically, the present invention relates to an apparatus capable of quick heating.multidot.quick cooling.
2. Description of the Prior Art
A heat treating apparatus of particular interest to the present invention is disclosed in, for example, Semiconductor World 1985, 12, p 65. FIG. 1 illustrates the apparatus disclosed therein. A furnace having small heat capacity is employed as an apparatus for heat treating the substrate. The reason for this is that the smaller the heat capacity, the shorter the time required for heating and cooling. Therefore, reducing heat capacity improves process efficiency.
FIG. 1 illustrates a known apparatus for heat treating substrates. The apparatus comprises a furnace core tube 102 which is formed of quartz. Substrates 101 are contained within the tube 102. The apparatus further comprises a nichrome wire heater 103 provided around an outer circumference of the furnace core tube 102; an outer cylindrical furnace 105 (formed of a material with small heat capacity and a blower 113 for forcing cooling air to the space between the furnace core tube 102 and the outer cylindrical furnace 105. The furnace core tube 102 and the heater 103 are contained within the furnace 105. The apparatus is structured such that the inside of the furnace can be quickly heated or cooled.
The outer cylindrical furnace 105 is formed of a material having small heat capacity. Therefore, the ability of the apparatus to accumulate heat is small. Thus, the inside of the furnace can be quickly cooled. However, the apparatus shown in FIG. 1 has a disadvantage in that when a high temperature is maintained within the furnace the loss of heat is large since the apparatus has little heat insulation. Therefore, the above described apparatus is not preferred for high temperature heat treatment. By "high temperature heat treatment" is meant treatment performed at a temperature higher than, for example, 400.degree. C.
FIG. 2 illustrate an improvement of the apparatus shown in FIG. 1. The apparatus of FIG. 2 is used for heat treatment at temperatures of about 800.degree. C. to 1000.degree. C., for example. The apparatus shown in FIG. 2 comprises a mirror 108 for reflecting heat rays. The mirror 108 is formed on an internal circumferential surface of the outer cylindrical furnace 105 and is designed to reflect radiation heat.
In the furnace shown in FIG. 2, the mirror 108 becomes hot due to heat conduction through a gas existing between the heater 103 and the mirror 108. Therefore, the mirror 108 is formed of gold (Au) to prevent degradation of the mirror's ability to reflect heat and to prevent damage caused by heat. However, gold does not have a very high reflectance in the same wavelength range whereat the semiconductor substrate readily absorbs heat. Therefore, heat is inefficiently used in the prior art apparatuses.